This invention relates to an oscilloscope for the display of sectional planes of a body by means of forming an image of the echo impulses received pursuant to a linear scanning of the body with ultrasonic impulses, with a line rate deflecting system for producing a magnetic deflection field of an image display tube, the active line trace being triggered by a control impulse produced in dependence upon a transmitted ultrasonic impulse.
A prior art oscilloscope of this type is characterized by a line rate deflecting system comprising a total of two circuits for supplying current to the line deflection coil. The first circuit with a first voltage source delivers a constant biasing current of such polarity that, due to the magnetic field of the line deflection coil, the electron beam of the display tube is retained in the non-triggered state in an inoperative position on the left side of the tube picture screen. In contrast thereto, the second circuit serves the purpose of producing the line synchronizing impulse in the following manner. With the occurrence of a trigger impulse, a transistor is actuated to the conductive state, and the line deflection coil is thereby connected to a second voltage source having a polarity opposite that of the voltage source for the biasing current. The linear rise in current in the deflection coil as effected thereby, in superposition with the biasing current, then represents the ascending flank of the sweep current impulse for horizontal deflection of the electron beam by means of the corresponding magnetic field of the line deflection coil. In contrast thereto, the negative going flank of the current impulse which returns the electron beam to the initial rest position results by blocking of the transistor due to current interruption by means of a diode. However, in oscilloscopes of the above type, there are further essential requirements in addition to the triggerability of the line sweep. One of these requirements is that in order to preserve the linearity of the image display or recording, the respective line deflection must proceed with absolutely uniform speed over the entire height or extent of the picture, independently of varying transit paths of the electron beam to the tube picture screen (the transit paths being longer at the screen edges than in the center of the screen). An additional requirement is that, in the interest of close line intervals (optimum line frequency for the purpose of an optimum image resolution), the return time of the electron beam to the initial position, that is the return sweep period of the deflection generator, should be as brief as possible. Finally, the entire deflection system should exhibit the lowest possible power loss. Practice has shown that a line-sweep coil deflection system such as is used in oscilloscopes of the above type cannot meet the requirements listed above to the degree desired. Due to the linearity of the rise in current of the line synchronizing impulse, only constant beam deflection speeds are yielded, on the one hand, which, however, are converted to non-uniform line deflection speeds on account of the varying transit paths of the electron beam to the tube picture screen. Line deflection proceeds more rapidly on the edges of the picture screen than in the center of the screen, so that the image points of echo impulses of each line, with the same chronological impulse intervals, succeed one another less rapidly toward the edges of the picture screen than in the center of the picture screen. On the other hand, in the known deflection system, the return sweep period is restricted to finite lower boundary values by time constants which are determined by coil inductance and non-negligible ohmic electric circuit resistances, and it is not possible to obtain a return sweep period below that determined by these time constants. Finally, as a consequence of a constantly flowing, relatively high biasing current, an unnecessarily high power loss also results.